The Islamic State group (IS) claimed the attack, which came after a series of bloody extremist assaults this week, in the town of Sehwan in Sindh province, some 200 kilometres (124 miles) northeast of the provincial capital Karachi.

"Both the federal and provincial law enforcement authorities and police started a crackdown across the country before dawn, and scores of suspects have been arrested from different cities," a government official speaking on condition of anonymity told AFP.

He said the sweeping operation will continue for the coming days.

A statement from the paramilitary Rangers said at least 18 terrorists had been killed in operations in Sindh province overnight, while police in the city of Peshawar said seven more had been killed in the country's northwest.

Police had cordoned off the shrine of Lal Shahbaz Qalandar, a revered 13th century Muslim saint, early Friday, as forensic investigators arrived.

The popular shrine's white floor was still smeared with blood, with scattered debris including shoes, shawls, and baby bottles. At least 20 children are believed to be among the dead, the head of Sehwan's medical facility Moeen Uddin Siddiqui said.

At 3.30 am the shrine's caretaker stood among the carnage and defiantly rang its bell, a daily ritual that he vowed to continue, telling AFP he will "not bow down to terrorists".

The Sindh provincial government announced three days of mourning as Pakistanis vented their grief and fury on social media, bemoaning the lack of medical facilities to help the wounded, with the nearest main hospital some 130 kilometres from the shrine.

The medical facilities in Sehwan are basic, and many of the injured were flown to Karachi and other major towns of Sindh in military planes and helicopters.

"Sunnis, Shias, Hindus, ppl from all faiths visit Sehwan often to pay homage to the great saint. This is an attack on our identity & culture," said Twitter user Zahraa Saifullah.

"Did india or afghanistan tell us to not build hospitals to deal with this type of situation," wrote Nizam Baluch about the lack of medical facilities.

Pakistan has seen a dramatic improvement in security in the past two years,but multiple attacks this week -- most claimed by the Pakistani Taliban -- has undermined the growing sense of optimism.

The assaults underscore Pakistan's struggle to stamp out extremism, which was stepped up after the country's deadliest ever attack, a Pakistani Taliban assault on a school in Peshawar in 2014 which left more than 150 people dead -- mostly children.